1) Field of the Invention
This invention relates to a brake band suitable for use in a transmission or like of a vehicle such as an automobile, an agricultural machinery or a construction machinery.
2) Description of the Related Art
In an automatic transmission, multiplate clutches, brake bands and the like are used to effect speed changes by causing an input element such as a gear train to engage or by releasing the same. Of these, brake bands are widely used for their preferred characteristic that, even if the same acting force is applied to a drum, their braking force differs depending on the braking direction of the drum and smooth up-shifting is feasible.
A fundamental construction of a brake band is illustrated in FIG. 3. The brake band, which is generally indicated at numeral 7, is formed of a curved strap 3 made of a thin steel plate. Brackets 4,4xe2x80x2 are connected to opposite end portions of the strap 3, respectively. A porous thin friction material (lining) 2 is bonded with an adhesive on an inner peripheral surface of the curved strap 3.
Of these brackets, the bracket 4 is an apply-side bracket on which hydraulic pressure for actuating the brake acts, whereas the bracket 4xe2x80x2 is an anchor-side bracket.
To improve the oil discharge characteristic between the drum and the lining in the course of a binding operation of the brake band in such a band brake, the lining 2 may be provided with an oil groove 5, which may in turn be provided with oil discharging apertures 6.
The action of the brake band will next be described in short with reference to FIG. 4. The brake band encircles a drum 1. When it is desired to actuate the brake, hydraulic pressure is applied as indicated by arrow P to the bracket 4 on an apply side to bind the brake band. Designated at 4xe2x80x2 is the anchor-side bracket on an opposite end, that is, on a support side.
A direction of rotation of the drum indicated by arrow L in the drawing is called a xe2x80x9cleading directionxe2x80x9d, while a direction of rotation in its opposite direction is called a xe2x80x9ctrailing directionxe2x80x9d.
If the arrangement of the oil groove 5 and the oil discharging apertures 6 is not appropriate and the discharging function of the brake band for an oil film existing between the drum and the lining in the course of binding of the brake band is low, the brake band exhibits a low friction characteristic especially during an initial period of binding in which the pressing force is low. This leads to slipping tendency in an initial stage of binding and then to a sudden increase in binding force in a final stage of binding, resulting in an increased shift shock. If the discharge of oil is excessive conversely, starting friction torque in the initial stage of binding becomes large and the shifting performance is deteriorated. Accordingly, a variety of ideas have been proposed with respect to the arrangement of an oil groove and oil discharging apertures.
Further, when a drum is rotating in a leading direction, a surface pressure applied on the anchor side end is generally at least twice as much as a surface pressure applied on the apply side. More heat is therefore produced on the anchor side especially when the brake band is used at elevated temperatures where the viscosity of oil becomes lower. This makes it impossible to maintain an oil film on the anchor side. The brake lining is therefore caused to burn out little by little at its surface. As the brake lining is longer, greater torque is produced in the initial stage of binding, thereby causing an increase in shift shock.
To cope with the above-mentioned problem, an arrangement of oil grooves and oil discharging apertures such as that shown by way of example in FIG. 5 has been invented. In this drawing, a left-hand end of a lining 2 is shown as an apply side whereas its right-hand end is indicated as an anchor side (which are designated as AP and AC, respectively). In the illustrated example, apply-side oil grooves 8 are provided with oil discharging apertures 9, respectively. On the other hand, anchor-side oil grooves 8xe2x80x2 are not provided with oil discharging apertures and serve as oil reservoirs. This arrangement of the oil grooves and oil discharging apertures not only reduces the amount of oil to be discharged but also provides oil reservoirs in the vicinity of the anchor-side end while minimizing a reduction in the number (i.e., area) of grooves and discharging apertures as friction surface cooling means to as great as an extent as possible.
The arrangement shown in FIG. 5 is an example, and a variety of arrangements are being adopted depending on the purpose and manner of use of the brake band.
It has heretofore been the practice to concurrently conduct the formation of oil grooves by punching on a press or by cutting with a laser or a cutter in the course of fabrication steps of a lining prior to bonding the lining onto a. strap, that is, in the course of steps comprising impregnating a green paper sheet with a resin, drying the thus-impregnated green paper sheet, heating the thus-dried green paper sheet to form a lining paper sheet, and then cutting the lining paper sheet in a product size. Thereafter, the lining is bonded onto the strap, which is made of a thin steel plate or the like, with an adhesive, whereby a brake band is completed.
In such a conventional brake band, a lining 2 is bonded on a strap 3, and grooves 8 have been formed by punching or the like, as shown in FIG. 6. A resin film is formed on a surface of the lining 2. According to the conventional formation of the grooves by punching, however, the surface resin film and fibers of the lining are both cut off as can be seen from FIG. 6.
As a result, oil tends to leak out through interstices between cut fibers in cut side walls 81 of each groove 8 when the brake band is pulled tight to apply braking force, thereby failing to form a sufficient oil film and hence resulting in the occurrence of a shift shock. Even when friction surfaces are supplied with oil overflowed from the grooves, upper corner portions 82 of each groove are formed into acute edges when the lining 2 is brought into contact with the brake drum 1 (see FIG. 4). This involves a potential problem that the upper corner portions 82 may cut off a flow of oil supplied from the corresponding groove to the friction surfaces. Moreover, with the oil grooves extending through the lining, it is difficult to produce sufficient oil film pressure under the supply of lubricating oil in a small quantity. Cutting of an oil film leads to a problem that starting friction torque becomes higher in the initial stage of tightening of the brake band and a shift shock is hence produced.
With the foregoing in view, the present invention has a primary object thereof the provision of a brake band comprising a strap and a lining bonded on an inner peripheral surface of the strap, wherein the lining is provided in a surface thereof with at least one oil groove formed under compression to a predetermined depth.
Since the oil groove has been formed under compression in the lining of the brake band according to the present invention, fibers of the lining are not cut off around the oil groove. Oil is therefore prevented from leaking out of the lining through side walls of the oil groove. Further, the depth of the groove can be chosen as desired.
Preferably, upper corner portions of the oil groove can be formed into curved surfaces. This makes it possible to avoid the problem that a flow of oil may be cut off when the brake band is pulled tight to apply braking force.